Because of their high energy density, lithium ion secondary batteries are increasingly used in recent years as portable power sources for laptop computers, mobile phones, digital cameras and the like. Also great efforts are devoted to the development of lithium ion secondary batteries as power sources for electric automobiles which are desired to reach a practically acceptable level as environment-friendly automobiles.
The lithium ion secondary batteries, albeit their high performance, are not satisfactory with respect to discharge characteristics in a rigorous environment, especially low-temperature environment, and discharge characteristics at high output levels requiring a large quantity of electricity within a short duration of time. Most batteries use electrolytic solutions based on low-flash-point solvents, typically dimethyl carbonate and diethyl carbonate. In case of thermal runaway in the battery, there is a risk of ignition. An improvement in safety is desired.
Reference should be made to JP-A 11-214032, JP-A 2000-58123 both corresponding to U.S. Pat. No. 6,124,062, JP-A 2001-110455, and JP-A 2003-142157.